Shark Skin Inspires Ship Coating

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Shark Skin Inspires Ship Coating

A new environmentally friendly coating based on sharks' skin may soon help the U.S. Navy increase ship speeds while saving fuel.

The coating doesn't go anywhere near the engines – it will be applied on the hull of ships below the waterline, where all manner of algae, barnacles and other wee beasties attach themselves, slowing ships and reducing their maneuverability.

"Aircraft carriers can't get up enough speed to launch their planes if there's too much marine growth on their hulls," says Anthony Brennan, a University of Florida professor of materials science and engineering and the lead developer of the shark-inspired coating.

Putting an aircraft carrier into dry dock to clean the hull is expensive and time-consuming, said Brennan. Water inlets for nuclear-powered ships and submarines are also constantly getting plugged up with algae, barnacles and other matter.

"Navy ships spend the majority of their time in harbors, and that's when marine growth can really build up," he said.

Of the $550 million to $600 million the Navy spends annually on powering its ships and submarines, at least $50 million stems directly from drag due to marine growth fouling the vessels' hulls, said Stephen McElvany, an environmental quality program officer in the Office of Naval Research's physical science division.

Existing antifouling paints such as tributyltin, or TBT, kill algae and barnacles when they latch on. TBT is being banned worldwide by the International Maritime Organization, or IMO, the U.N. body responsible for overseeing shipping-related issues.

TBT has caused deformations in oysters and sex changes in snails, according to Susan Sang, a wildlife toxicologist for the World Wildlife Fund in Canada. The chemical also accumulates in fish, sea birds and marine mammals, damaging reproductive and immune systems, Sang said.

Although military vessels are exempt from the IMO ban, the Navy stopped using TBT many years ago in favor of copper-based paints that also kill organisms that latch on.

"Copper-based paints are less harmful than TBT, but are still toxic," said Sang.

The Navy is funding research at the University of Florida in hopes of obtaining a nontoxic coating that will reduce routine cleaning of fouled ships, McElvany said. Currently, every one or two years divers operate specially designed scrubbing machines and clean ships while they're in the water.

"It's a costly operation," McElvany said.

To find a way to persuade algae to move on rather than killing them, Brennan and colleagues turned to nature. Sharks don't have algae or barnacle problems despite being underwater all their lives. Shark skin is made up of tiny rectangular scales topped with even smaller spines or bristles. This makes shark skin rough to the touch. This irregular surface makes it difficult for plant spores to get a good grip and grow into algae or other plants.

"It's like trying to walk across a bed of nails when some nails are longer and unevenly distributed," Brennan said.

Using a combination plastic-and-rubber coating, Brennan replicated a version of shark skin that is made up of billions of tiny raised, diamond-shaped patterns, visible under a microscope. Each "sharklet" diamond measures 15 microns, or 15 thousandths of a millimeter, and contains seven raised ribs that resemble different lengths of raised horizontal bars.

In lab tests, the coating – provisionally named Gator Sharkote – reduced by 85 percent the settlement of spores from a very common and detrimental type of algae called Ulva, a green seaweed often seen on the sides of ships.

"The only place the spores land right now is where we have a defect in the pattern," Brennan said.

The coating makes use of the fact that algae spores actually check out a surface before gluing themselves on. McElvany calls the coating "kind of magical" because the spores don't like the pattern and move on.

But one pattern might not be good enough for all types of marine growth.

"Marine plant spores are unbelievably adaptive and will change their shape to attach on to surfaces," said Brennan.

Sharks cope with this easily enough because their scales move and flex as they swim. In research recently patented, Brennan and his colleagues have made the diamond-shaped pattern dynamic, or changeable, under the influence of a low-power electric current.

The ribs on the surfaces swell and shrink, in effect flexing in and out from the hull surface as the current varies. That may also be useful because the movement could prevent the accumulation of silt and other debris on the hulls, which is often a precursor to plant and barnacle growth, he said.

The new, dynamic version of Gator Sharkote is currently being put to the test against critters found in oceans off Florida, England, Hawaii, California and Australia. Coating Navy vessels with some version of Gator Sharkote is still many years away, said McElvany, but the results from the basic research so far have been "pretty amazing."

There may also be important biomedical applications, Brennan said. Testing has shown that the dynamic version of the coating impedes the attachment and growth of animal cells, which may make it useful on medical implants such as catheters and heart valves. Currently, cell and tissue growth on these implants often reduces or impedes their function.

"Our whole concept is a surface design that we can tailor to the application, whether in the ocean or human body," Brennan said.